This invention relates to a sync or synchronizing signal generator, and particularly to a sync generator for a color television system applicable to an NTSC or PAL-M system.
It is well known to produce television sync pulses such as, for example, horizontal and vertical sync pulses, by counting down a carrier signal. A carrier signal having a frequency of four times a color sub-carrier frequency f.sub.sc may be counted down to produce a color sub-carrier signal S.sub.sc having a frequency of f.sub.sc and further counted down to produce horizontal and vertical sync signals.
When the power is first turned ON in such a sync generator, the counters thereof may come up in any condition and the relative phases of the color sub-carrier and synchronizing signals begin at random.
As is well known, in the NTSC color television signal, there is an exact frequency relationship between the horizontal synchronizing frequency f.sub.h and the color subcarrier frequency f.sub.sc, namely f.sub.sc =455/2f.sub.h, and consequently four television fields must occur before the color subcarrier signal exactly repeats itself in phase with respect to the horizontal synchronizing signal. In other words, the periodicity of the color frame is four fields. Assuming that the color subcarrier signal has is positive peak value at the front edge of the horizontal synchronizing signal, the color subcarrier signal has its negative peak value at the front edge of the next horizontal synchronizing signal. This means that the phase of the subcarrier signal is reversed at every horizontal interval. As a result of the reversal, if the subcarrier has its negative peak value at the front edge of the first equalizing pulse included in the first field of a first frame, the positive peak value of the subcarrier signal occurs at the front edge of the first equalizing pulse included in the first field of the next frame, which immediately succeeds the first frame. In that sense, these two frames are different. It will be evident that if a continuous signal is to be reproduced, splices must join successive frames in the correct sequence; i.e., the first frame must be joined to the second frame. If the first frame is joined to another frame having the same phase relationship between the color subcarrier and the first equalizing pulse as the first frame so as to constitute another "first" frame, there will be a sudden 180.degree. phase shift in the burst or clor subcarrier signals at the splicing point.
The phase of the vertical and horizontal synchronizing signals changes from field to field. In the first field in a frame, the vertical and horizontal synchronizing signals substantially coincide, whereas in the second field in a frame, the vertical synchronizing signal is offset by about 0.5 H where H is a horizontal interval. As described in the in the preceding, an NTSC color television signal consists of four fields in two consecutive color frames sequentially repeating. The first frame, color frame A, contains color field I and color field II, the second frame, color frame B, contains color field III and color field IV. The vertical synchronizing signals of odd-numbered color fields (I and III) employ vertical and horizontal synchronizing signals substantially aligned with each other whereas the even-numbered color fields (II and IV) have the vertical synchronizing signal shifted 0.5 H with respect to the horizontal synchronizing signal as necessary to produce interlaced scanning. The phase of the color sub-carrier signal is the same in color fields II and III and also the same in color fields IV and I. However, the phase of the color sub-carrier in fields II and III is 180.degree. out of phase with the phase of the color sub-carrier in fields I and IV. A PAL-M color signal employs four consecutive frames (eight consecutive fields) sequentially repeating instead of the four used in an NTSC system.
When signals from more than one source such as, for example, a video tape recorder and a television camera, are combined for editing purposes, the editing apparatus requires a color framing signal indicating a starting point in the sequence of color fields (four in NTSC and eight in PAL-M) in order to maintain color synchronization through an editing point.
Merely counting down the vertical frequency to 1/4 f.sub.v (for NTSC) or 1/8 f.sub.v (for PAL-M) is not satisfactory for such a color framing signal since, as a result of synchronizing uncertainties arising when the equipment is turned ON, the phase relationships between the vertical and horizontal synchronizing signals as well as between the color sub-carrier signal produced by counting down a carrier signal 4 f.sub.sc by 1/4 and such a counted down vertical frequency, are not determined.